Embodiments relate generally to marine vibrators for marine geophysical surveys. More particularly, embodiments relate to a piezoelectric bender that comprises a spring and mass element to provide additional constructive resonance.
Sound sources are generally devices that generate acoustic energy. One use of sound sources is in marine seismic surveying in which the sound sources may be employed to generate acoustic energy that travels downwardly through water and into subsurface rock. After interacting with the subsurface rock, e.g., at boundaries between different subsurface layers, some of the acoustic energy may be returned toward the water surface and detected by specialized sensors (e.g., hydrophones, geophones, etc.). The detected energy may be used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
One example of a sound source includes a bender as the active part. This type of sound source is typically referred to as a “piezoelectric bender” because it uses the piezoelectric effect to generate acoustic energy. A piezoelectric bender may include a base plate of elastic material (e.g., aluminum) and a piezoelectric body attached to the base plate. When an electrical field is applied across the composite assembly of the base plate and the piezoelectric body, the composite assembly should bend and thus create vibrations from the composite assembly to a fluid (e.g., water). The piezoelectric bender may have a first resonance frequency as a result of the composite assembly functioning as a spring, together with the surrounding oscillating water mass.
FIG. 1 shows a finite-element analysis of an axial-symmetric model of a piezoelectric bender 100 working close to its first resonance. As illustrated, the piezoelectric bender 100 may comprise a base plate 105 and piezoelectric body 110, which may bend from a first position to a second position shown at 105′ and 110′, respectively. The axial line of symmetry for the piezoelectric bender 100 is represented by reference number 102. FIG. 2 is a graph showing an example of far-field intensity as a function of frequency for the piezoelectric bender 100 of FIG. 1. As illustrated by FIG. 2, the piezoelectric bender 100 may have a sharp resonance peak, causing the bandwith of this design to be limited.